Method for measuring sheet material flatness and method for producing steel sheet using said measuring method

ABSTRACT

A method for measuring flatness of a sheet material in which a light and dark pattern composed by a light portion and a dark portion is projected onto a surface of the sheet material running in a longitudinal direction, a pattern image is acquired by photographing the light and dark pattern by an image pickup device having an image pickup visual field larger than a width of the sheet material, and the flatness of the sheet material is measured by analyzing the acquired pattern image. A staggered pattern is used for the projecting step and for light to be specularly reflected for receipt by the image pickup device. Calculating the flatness also includes steps of setting a shape measurement line, averaging picture element concentrations, calculating a distribution of the concentrations, and calculating the flatness based on surface shape using the distribution.

TECHNICAL FIELD

The present invention relates to a method for measuring, with highaccuracy, flatness of a sheet material such as a steel sheet running inthe longitudinal direction, and a method for producing a steel sheetusing said measuring method.

BACKGROUND ART

A sheet material is required to have high flatness for assuring qualityand for consistent production. In fulfilling this requirement, how theflatness is to be controlled properly in the production process of sheetmaterial has conventionally been a challenge.

Generally, as an index indicative of flatness, difference in elongationpercentage or degree of steepness is used.

The difference in elongation percentage Δε is a difference between anelongation percentage ε_(CENT) in a central portion in the widthdirection of a sheet material and an elongation percentage ε_(EDGE) in aportion other than the central portion in the width direction of thesheet material (generally, in the vicinity of the edge) as measured in acertain section in the longitudinal direction of the sheet material, andis represented by Formula (2).Δε=ε_(CENT)−ε_(EDGE)  (2)

Also, the degree of steepness λ is defined as λ=δ/P by using the heightδ of standing wave of sheet and the pitch P thereof. In the case wherethe shape of the standing wave of sheet is approximated as a sinusoidalwave, the well-known relationship represented by Formula (3) existsbetween the difference in elongation percentage Δε and the degree ofsteepness λ (%).

$\begin{matrix}{\lambda = \left\{ \begin{matrix}{{+ \frac{2}{\pi}}{{\Delta ɛ}}^{1/2} \times 100\mspace{14mu}\left( {{{when}\mspace{14mu}{\Delta ɛ}} \geqq 0} \right)} \\{{- \frac{2}{\pi}}{{\Delta ɛ}}^{1/2} \times 100\mspace{14mu}\left( {{{when}\mspace{14mu}{\Delta ɛ}} < 0} \right)}\end{matrix} \right.} & (3)\end{matrix}$

For example, the production line for a hot-rolled steel sheet, which isone example of the sheet material, generally comprises a heatingfurnace, roughing mill, finishing mill train, cooling zone, and coilingmachine. A starting material heated in the heating furnace is rolled bythe roughing mill to produce a slab (rough bar) having a thickness of 30to 60 mm. Next, the slab is rolled by the finishing mill trainconsisting of six to seven finishing mills to produce a hot-rolled steelsheet having a thickness required by the customer. This hot-rolled steelsheet is cooled by the cooling zone, and is coiled by the coilingmachine.

The production of a hot-rolled steel sheet having high flatness isimportant for ensuring the product quality, for stably passing the steelsheet through the finishing mill and coiling it by using the coilingmachine, and also for maintaining high productivity. Poor flatness of ahot-rolled steel sheet is caused by unevenness in the sheet widthdirection of elongation percentage produced in the finishing mill trainand cooling zone. Therefore, as a method for producing a hot-rolledsteel sheet having high flatness, there has been proposed a method inwhich a flatness meter or a sheet thickness profile meter is installedbetween the finishing mills or on the exit of the finishing mill train,and based on the measured value of the meter, the work roll bender ofthe finishing mill is feedback controlled, or a method in which thesetup condition of the shift position of work roll or the loaddistribution of finishing mill train is controlled by learning. Theabove-described controlling method is described, for example, inJP11-104721A. Also, there has been proposed a method in which a flatnessmeter is installed on the exit of the cooling zone, and based on themeasured value thereof, the amount of cooling water of cooling nozzlesof the cooling zone is feedback controlled. To carry out theabove-described controlling method, a method and a device for measuringthe flatness of a hot-rolled steel sheet running at a high speed at alocation between the finishing mills, on the exit of the finishing milltrain, or on the exit of the cooling zone have been devised and used foractual machines.

As a conventional method for measuring the flatness of a hot-rolledsteel sheet, there has been known a method in which a linear patternconsisting of a plurality of light lines extending in the sheet widthdirection is projected onto the surface of the hot-rolled steel sheetthat has been hot rolled and is running, the linear pattern isphotographed from the direction different from the linear patternprojecting direction by a two-dimensional camera, and based ondistortion of the linear pattern in the picked-up image, the surfaceshape, and therefore the flatness of the hot-rolled steel sheet ismeasured. In this method, by projecting the linear pattern over therange of about 1 m in the longitudinal direction (rolling direction) ofthe hot-rolled steel sheet, the measurement accuracy in the state inwhich standing waves of sheet observed frequently at a location justclose to the exit of the finishing mill exist steadily (at the fixed endbecause the standing waves of sheet are fixed by the finishing mill) isrestrained from being deteriorated. The above-described flatnessmeasuring method is described, for example, in JP61-40503A andJP2008-58036A.

JP61-40503A describes a method in which by scanning three laser beams,which are fired spacedly in the longitudinal direction of sheet, at ahigh speed in the sheet width direction, a linear pattern consisting ofthree light lines is projected onto the sheet surface, this linearpattern is photographed by a camera, and based on the distortion of thelinear pattern in the picked-up image thus obtained, the surface shape,and therefore the flatness of the sheet is measured. However, the linearpattern consisting of three light lines does not allow highly accuratemeasurement of the sheet surface shape, so that there arises a problemthat the measurement accuracy is extremely deteriorated especially whenthe period of standing waves of sheet is short.

JP2008-58036A describes a method in which a high-density linear patternconsisting of a plurality of light lines extending in the sheet widthdirection is projected onto the sheet material surface by using a slideon which the high-density linear pattern is drawn, this linear patternis photographed by a camera, and based on the distortion of the linearpattern in the picked-up image thus obtained, the surface shape, andtherefore the flatness of the sheet material is measured. In thismethod, unlike the method described in JP61-40503A, projection of thehigh-density linear pattern increases the measurement resolution (spaceresolution) of the surface shape, so that it can be expected that thesurface shape of the sheet material can be measured with high accuracy.

The shape measuring method as described in JP2008-58036A is generallycalled a grating pattern projection method, and is used widely invarious applications, being not limited to the case where the surfaceshape of a steel sheet is measured.

FIG. 1 is a schematic view showing a configuration example of a devicefor carrying out the grating pattern projection method. As shown in FIG.1, in the grating pattern projection method, a grating pattern isprojected onto the sheet material surface from the slantwise upper sidewith respect to the sheet material surface by a projector provided witha light source, a slide on which the grating pattern (generally, alinear pattern) is drawn, and an imaging lens. Then, the grating patternprojected onto the sheet material surface is photographed from thedirection different from the grating pattern projecting direction by atwo-dimensional camera. At this time, if the surface shape of the sheetmaterial changes, an inclination angle of the sheet material surfacealso changes, and the pitch of the grating pattern in the picked-upimage photographed by the camera (generally, the space between the lightlines composing the linear pattern) changes according to the inclinationangle of the sheet material surface. The relationship between theinclination angle of the sheet material surface and the pitch of thegrating pattern in the picked-up image can be determined geometrically.Therefore, if the pitch of the grating pattern in the picked-up image ismeasured, the inclination angle of the sheet material surface can becalculated based on this measurement result and the above-describedrelationship. If the calculated inclination angle is integrated, thesurface shape of the sheet material can be calculated.

SUMMARY OF INVENTION

In the case where the surface shape, and therefore the flatness of ahot-rolled steel sheet is measured by using the grating patternprojection method, as described above, a linear pattern consisting of aplurality of light lines extending in the sheet width direction isprojected onto the steel sheet surface as the grating pattern. Then, ashape measurement line extending along the longitudinal direction of thehot-rolled steel sheet is set at a position at which the surface shapemust be measured to calculate the flatness in the picked-up image of thelinear pattern, and based on the picture element concentrationdistribution on the shape measurement line, the distribution of thepitches in the linear pattern on the shape measurement line (the spacebetween the light lines composing the linear pattern) is calculated.Next, based on the distribution of the pitches in the linear pattern onthe shape measurement line, the distribution of inclination angles ofthe steel sheet surface on the shape measurement line is calculated, andby integrating this inclination angle along the shape measurement line,the surface shape of the steel sheet on the shape measurement line iscalculated. Further, based on the calculated surface shape, the flatnessis computed.

In the case where a device for carrying out the grating patternprojection method as shown in FIG. 1 is installed on the production linefor a hot-rolled steel sheet, and the finishing mill is controlled byfeeding back the measured flatness value in real time, the device mustbe installed at a location just close to the exit of the finishing mill.At the location just close to the exit of the finishing mill, asufficient installation space for the device cannot be secured in manycases because measuring instruments such as a sheet thickness meter, asheet width meter, and a sheet thermometer are installed, and besidesthe cooling zone for water cooling is installed just close to thislocation.

In order to reduce the installation space for the device as far aspossible, it is thought that, firstly, the projector and the camera arebrought closer to the hot-rolled steel sheet to reduce the installationspace in the vertical direction, and secondly, the angles of view of theprojector and the camera are set so as to be on the wide side so thatthe measurement range (about 1 m in the longitudinal direction) of thehot-rolled steel sheet enters within the angle of view of projection ofthe projector and within the angle of view of the camera. However, asshown in FIG. 2, in the case where the angle of view of projection ofthe projector is wide, in order to reduce the installation space in thehorizontal direction, the camera must be arranged at a position at whichthe specularly reflected light of projected light of the projector (thespecularly reflected light of linear pattern) can be received. From theviewpoint of enhancing the measurement resolution (space resolution) ofthe surface shape, a linear pattern having a small pitch has only to beprojected. However, since the surface of the hot-rolled steel sheetimmediately after being finish rolled has a strong specular reflectionproperty (the reflection intensity of specular reflection component ishigh), if the camera is arranged at a position at which the specularlyreflected light of projected light of the projector can be received, theoutput signal sent from an element receiving the specularly reflectedlight of the light receiving elements of the camera saturates, andhalation occurs. Therefore, in the picture element region ofphotographed images corresponding to the element receiving thespecularly reflected light and the peripheral elements, the adjacentlight lines stick to each other, and the linear pattern is liable tocollapse. Also, if the sensitivity of the camera is reduced excessivelyto prevent the linear pattern from collapsing, the output signalintensity of elements other than the element receiving the specularlyreflected light becomes insufficient, so that the concentration ofpicture element corresponding to the element having an insufficientoutput signal intensity in the photographed image decreases, andtherefore a linear pattern in which the light lines are difficult todistinguish is formed.

The present invention has been made to solve the problems with the priorarts explained above, and accordingly an objective thereof is to providea method for measuring the flatness of a sheet material such as a steelsheet running in the longitudinal direction, in which even in the casewhere an image pickup device is arranged at a position at which thespecularly reflected light of a light and dark pattern projected ontothe surface of a sheet material having a strong specular reflectionproperty can be received, the surface shape of the sheet material can bemeasured with high accuracy, whereby the flatness of the sheet materialcan be measured with high accuracy.

In the case where the light and dark pattern projected onto the surfaceof a sheet material is made the linear pattern having a small pitch, ifthe image pickup device is arranged at a position at which thespecularly reflected light can be received, as the countermeasures forpreventing the collapse of the linear pattern in the picture elementregion corresponding to the element receiving the specularly reflectedlight and the peripheral elements, the methods described below areconceivable: (1) a method in which a camera having a wide dynamic rangeis employed as the image pickup device so that even if the sensitivityof the image pickup device is reduced, the output signal intensity ofthe element that does not receive the specularly reflected light doesnot become insufficient, and (2) a method in which the pitch of thelinear pattern is increased.

However, although a dynamic range of 12 bits (4096 gradation) or largercan be obtained by a digital camera that has come into wide userecently, the countermeasure of item (1) has a problem of restrictedwiring length and increased camera cost, and therefore cannot be appliedeasily in some case.

Also, in the countermeasure of item (2), if the pitch of the linearpattern is increased simply as shown in FIG. 3B, the measurementresolution (space resolution) of the surface shape decreases, whichleads to deterioration in the measurement accuracy of the surface shape,and therefore the measurement accuracy of the flatness.

Accordingly, the present inventors conducted studies earnestly, and hitupon an idea that a staggered pattern shown in FIG. 3C is to be used asthe light and dark pattern projected onto the surface of the sheetmaterial, the staggered pattern being composed of light portionsarranged in a staggered form at a predetermined preset pitch (presetpitch P_(L) in the longitudinal direction, preset pitch P_(W) in thetransverse direction) in the longitudinal direction and the transversedirection, and this pattern is to be projected onto the surface of thesheet material so that the longitudinal direction of the staggeredpattern corresponds to the longitudinal direction of the sheet material,and the transverse direction thereof corresponds to the width directionof the sheet material. With this staggered pattern, since the lightportions are arranged in a staggered form in the longitudinal directionand the transverse direction, even if the longitudinal preset pitchP_(L) between light portions is equal to the preset pitch P_(L)′ of theconventional linear pattern (FIG. 3A), the distance between the lightportions adjacent in the longitudinal direction (for example, lightportions M1 and M2) becomes longer (or doubled) than the distance P_(L)′between the light portions adjacent in the longitudinal direction in theconventional linear pattern, and therefore the space between the lightportions increases. Concerning the transverse direction, the lightportion is continuous in the conventional linear pattern, whereas in thestaggered pattern, the light portions adjacent in the straight-line formin the transverse direction (for example, light portions M1 and M3) havea space therebetween. Therefore, the staggered pattern has an advantagethat even in the picture element region corresponding to the element orthe like of image pickup device receiving the specularly reflectedlight, the light and dark pattern is difficult to collapse.

However, even if the staggered pattern is used as the light and darkpattern projected onto the surface of the sheet material, if the surfaceshape of the sheet material is calculated simply based on thedistribution of concentrations of picture elements on a shapemeasurement line L1 extending along the longitudinal direction of thesheet material (the longitudinal direction of the staggered pattern) asin the conventional example, the space between the light portionsadjacent in the straight-line form in the longitudinal direction iswide, so that the measurement resolution (space resolution) of thesurface shape decreases.

Accordingly, the present inventors further conducted studies earnestly,and hit upon an idea that the concentrations of picture elements on astraight line L2 that extends in the transverse direction of thestaggered pattern passing through the picture element on the shapemeasurement line L1 and has a length W two times or more the transversepreset pitch P_(W) between the light portions are to be averaged, andthereby an average picture element concentration is to be calculated.For example, it is assumed that the picture element concentration of alllight portions of the staggered pattern is 254, and the picture elementconcentration of all dark portions thereof is zero. Assuming that thelength W of the straight line L2 is two times the transverse presetpitch P_(W) between the light portions (W=2P_(W)), and the number ofpicture elements of the light portion on the straight line L2 is equalto the number of picture elements of the dark portion, the averagepicture element concentration on the straight line L2 is 127. If thedistribution of average picture element concentrations along the shapemeasurement line L1 is calculated (the longitudinal position of thestraight line L2 is changed), this distribution of average pictureelement concentrations is a distribution in which the average pictureelement concentration at the position at which the straight line L2passes through the light portion is 127, and the average picture elementconcentration at the position at which the straight line L2 passesthrough the dark portion only is zero, that is, a distribution having aperiod that is the same as the longitudinal preset pitch P_(L) betweenlight portions. In other words, the period P_(L) of the distribution ofthe average picture element concentrations is equal to the period P_(L)′of the distribution of the picture element concentrations on the shapemeasurement line L′ for the conventional linear pattern (FIG. 3A).Therefore, if the surface shape of the sheet material is calculatedbased on the distribution of the average picture element concentrations,a measurement resolution of the same degree as the case where theconventional linear pattern is used can be obtained without a decreasein measurement resolution (space resolution) of the surface shape forthe longitudinal direction of the staggered pattern (the longitudinaldirection of the sheet material). The amplitude of distribution of theaverage picture element concentrations in the case where the staggeredpattern is used decreases as compared with the amplitude of distributionof the picture element concentrations in the case where the linearpattern is used. However, if the length W of the straight line L2 onwhich averaging is performed is made a length two times or more thetransverse preset pitch P_(W) between light portions, the light portionalways exists on the straight line L2. Therefore, the amplitude ofdistribution of the average picture element concentrations is aboutone-half the case where the linear pattern is used even if decreasingmost, which does not pose a problem. Also, the measurement resolution(space resolution) of the surface shape in the transverse direction ofthe staggered pattern (the width direction of the sheet material) doesnot pose a problem unless the length W of the straight line L2 isincreased extremely because the shape of the hot-rolled steel sheet,which is a main object to which the present invention is applied, doesnot change suddenly in the width direction although the measurementresolution decreases according to the length W.

As explained above, the present inventors reached a conclusion that ifthe surface shape of the sheet material is calculated by the proceduresof items (A) to (C) described below, even in the case where the imagepickup device is disposed at a position at which the specularlyreflected light of the light and dark pattern projected onto the surfacecan be received, the light and dark pattern is difficult to collapse,and the surface shape, and therefore the flatness of the sheet materialcan be measured with high accuracy without a decrease in measurementresolution.

(A) As the light and dark pattern projected onto the surface of thesheet material, the staggered pattern in which the light portions arearranged in a staggered form at the predetermined preset pitch in thelongitudinal direction and the transverse direction is used, and thestaggered pattern is projected onto the surface of the sheet material sothat the longitudinal direction of the staggered pattern corresponds tothe longitudinal direction of the sheet material and the transversedirection thereof corresponds to the width direction of the sheetmaterial.

(B) The concentrations of picture elements on the straight line thatextends in the transverse direction of the staggered pattern (the widthdirection of the sheet material) passing through the picture element onthe shape measurement line extending along the longitudinal direction ofthe staggered pattern (the longitudinal direction of the sheet material)and has a length two times or more the transverse preset pitch P_(W)between the light portions are averaged, and thereby the average pictureelement concentration is calculated.

(C) The distribution of the average picture element concentrations alongthe shape measurement line is calculated, and based on this distributionof the average picture element concentrations, the surface shape of thesheet material along the shape measurement line is calculated.

The present invention has been completed based on the above-describedfindings of the present inventors.

In order to achieve the objective, the present invention provides amethod for measuring flatness of a sheet material in which a light anddark pattern composed by a light portion and a dark portion is projectedonto a surface of the sheet material running in a longitudinaldirection, a pattern image is acquired by photographing the light anddark pattern by an image pickup device having an image pickup visualfield larger than a width of the sheet material, and the flatness of thesheet material is measured by analyzing the acquired pattern image,comprising the following first to sixth steps:

(1) First step: Using a staggered pattern as the light and dark patternprojected onto the surface of the sheet material, the staggered patternbeing composed of light portions arranged in a staggered form at apredetermined preset pitch in a longitudinal direction and a transversedirection, and projecting the staggered pattern onto the surface of thesheet material so that the longitudinal direction of the staggeredpattern corresponds to the longitudinal direction of the sheet materialand the transverse direction of the staggered pattern corresponds to thewidth direction of the sheet material.

(2) Second step: Arranging the image pickup device at a position atwhich the specularly reflected light on the surface of the sheetmaterial of the staggered pattern can be received, and acquiring thepattern image by photographing the staggered pattern by the image pickupdevice.

(3) Third step: Setting a shape measurement line extending along thelongitudinal direction of the staggered pattern at a predeterminedposition in the acquired pattern image.

(4) Fourth step: Averaging picture element concentrations on a straightline which passes through a picture element on the shape measurementline and extends in the transverse direction of the staggered pattern,and has a length two times or more the transverse preset pitch betweenthe light portions to calculate an average picture elementconcentration.

(5) Fifth step: Calculating distribution of the average picture elementconcentrations along the shape measurement line.

(6) Sixth step: Calculating a surface shape of the sheet material alongthe shape measurement line based on the calculated average pictureelement concentration distribution, and calculating the flatness of thesheet material based on the calculated surface shape.

In the present invention, the “preset pitch” means a value by which thespace between the light portions of the staggered pattern is projectedin the image pickup direction in the case where it is assumed that thesurface shape of the sheet material onto which the staggered pattern isprojected is completely flat. Especially, “longitudinal preset pitch”means a longitudinal space between the light portions adjacent in thestaggered form along the longitudinal direction of the staggeredpattern. Also, “transverse preset pitch” means a transverse spacebetween the light portions adjacent in the staggered form along thetransverse direction of the staggered pattern.

In the sixth step, in order to calculate the surface shape of the sheetmaterial along the shape measurement line based on the distribution ofthe average picture element concentrations along the shape measurementline, specifically, first, based on the distribution of the averagepicture element concentrations along the shape measurement line (forexample, by applying the publicly known phase analysis method to thedistribution of the average picture element concentrations), thedistribution p_(m)(x) of longitudinal pitches between light portions ofthe staggered pattern along the shape measurement line has only to becalculated. The relationship between the longitudinal pitch p_(m)between light portions of the staggered pattern and the inclinationangle θ of the sheet material surface can be determined geometrically.Therefore, if the distribution p_(m)(x) of longitudinal pitches betweenlight portions of the staggered pattern along the shape measurement lineis calculated, based on the distribution p_(m)(x) of longitudinalpitches between light portions and the above-described relationship, thedistribution θ(x) of the inclination angles of the sheet materialsurface along the shape measurement line can be calculated.

FIG. 4 is a schematic view showing the relationship between thelongitudinal pitch p_(m) between light portions of the staggered patternand the inclination angle θ of the sheet material surface. FIG. 4 showsan example in which the sheet material runs in the horizontal direction.In FIG. 4, θ denotes the inclination angle between the sheet materialrunning direction (horizontal direction) and the sheet material surface,α denotes the angle between the direction perpendicular to the sheetmaterial running direction (vertical direction) and the image pickupdirection of the image pickup device, and β denotes the angle betweenthe direction perpendicular to the sheet material running direction(vertical direction) and the projection direction of the staggeredpattern. Also, p_(m) denotes the longitudinal pitch between lightportions of the staggered pattern in the pattern image acquired for thesheet material, and p_(m0) denotes the value obtained by projecting thepitch p_(m) in the direction perpendicular to the sheet material runningdirection (vertical direction). Further, p_(s) denotes the longitudinalpitch between light portions of the staggered pattern in the patternimage acquired for a reference material that is placed in parallel withthe sheet material running direction (placed horizontally) and has aflat surface shape, and p_(s0) denotes the value obtained by projectingthe pitch p_(s) in the direction perpendicular to the sheet materialrunning direction (vertical direction).

Among θ, α, β, p_(m), p_(m0), p_(s) and p_(s0), Formulas (4) to (6) holdgeometrically.

$\begin{matrix}{{\tan\;\theta} = \frac{\left( {p_{m\; 0}/p_{{S\; 0}\;}} \right) - 1}{\left( {p_{m\; 0}/p_{S\; 0}} \right)\tan\;\beta}} & (4) \\{p_{S\; 0} = \frac{p_{S}}{\cos\;\alpha}} & (5) \\{p_{S\; 0} = \frac{p_{m}\cos\;\theta}{\cos\left( {\alpha - \theta} \right)}} & (6)\end{matrix}$

Substituting Formulas (5) and (6) into Formula (4), Formula (7) holds.

$\begin{matrix}{{\tan\;\theta} = \frac{\left( {p_{m}/p_{S}} \right) - 1}{{\tan\;\alpha} + {\left( {p_{m}/p_{S}} \right)\tan\;\beta}}} & (7)\end{matrix}$

From Formula (7), Formula (8) holds.

$\begin{matrix}{\theta = {\tan^{- 1}\left\{ \frac{\left( {p_{m}/p_{S}} \right) - 1}{{\tan\;\alpha} + {\left( {p_{m}/p_{S}} \right)\tan\;\beta}} \right\}}} & (8)\end{matrix}$

Therefore, the distribution θ(x) of the inclination angles of the sheetmaterial surface along the shape measurement line can be calculated fromFormula (1).

$\begin{matrix}{{\theta(x)} = {\tan^{- 1}\left\{ \frac{\left( {{p_{m}(x)}/{p_{S}(x)}} \right) - 1}{{\tan\;\alpha} + {\left( {{p_{m}(x)}/{p_{s}(x)}} \right)\tan\;\beta}} \right\}}} & (1)\end{matrix}$

Therefore, preferably, the method of the present invention furthercomprises a step of, for a reference material which is placed inparallel with the running direction of the sheet material, on whichflatness is measured, and has a flat surface shape, executing the firstto fifth steps to calculate the average picture element concentrationdistribution along the shape measurement line in the pattern imageacquired for the reference material, and based on the average pictureelement concentration distribution, calculating in advance thedistribution p_(s)(x) of longitudinal pitches between light portions ofthe staggered pattern along the shape measurement line in the patternimage acquired for the reference material; and the sixth step comprises:a step of, based on the average picture element concentrationdistribution calculated for the sheet material, calculating thedistribution p_(m)(x) of longitudinal pitches between light portions ofthe staggered pattern along the shape measurement line in the patternimage acquired for the sheet material, and a step of calculating thedistribution θ(x) of the inclination angles of the surface of the sheetmaterial along the shape measurement line based on Formula (1), andcalculating the surface shape of the sheet material based on thedistribution θ(x) of inclination angles of the surface of the sheetmaterial:

$\begin{matrix}{{\theta(x)} = {\tan^{- 1}\left\{ \frac{\left( {{p_{m}(x)}/{p_{S}(x)}} \right) - 1}{{\tan\;\alpha} + {\left( {{p_{m}(x)}/{p_{s}(x)}} \right)\tan\;\beta}} \right\}}} & (1)\end{matrix}$

In Formula (1), x denotes a position along the longitudinal direction ofthe staggered pattern in the pattern image (a position along thelongitudinal direction of the sheet material), θ(x) denotes thedistribution of inclination angles between the sheet material runningdirection and the surface of sheet material, α denotes the angle betweenthe direction perpendicular to the sheet material running direction andthe image pickup direction of image pickup device, and β denotes theangle between the direction perpendicular to the sheet material runningdirection and the projection direction of the staggered pattern.

In order to calculate the flatness of the sheet material, the shapemeasurement lines must be set at least in the central portion in thewidth direction of the sheet material and in a portion in the vicinityof the edge. However, in the case where the sheet material is, forexample, a hot-rolled steel sheet, the sheet material often runs in thestate in which meanders and a camber are produced. In this case, even ifthe width of the sheet material is fixed, the positional relationshipbetween the image pickup device and the sheet material edge changes inthe width direction of the sheet material. Therefore, if the shapemeasurement line is set at a coordinate fixed in the pattern imageacquired by the image pickup device, there arises a problem that theshape measurement line is not set correctly in the central portion inthe width direction of the sheet material on account of the meanders andcamber of the sheet material. In order to avoid this problem, it ispreferable that when the shape measurement line is set in the thirdstep, the picture element corresponding to the sheet material edge inthe pattern image acquired by the image pickup device is first detected,and the shape measurement line is set with the detected picture elementbeing a reference.

Therefore, preferably, the third step in the method of the presentinvention comprises: a step of setting an edge detection line extendingin the transverse direction of the staggered pattern at a predeterminedposition in the acquired pattern image; a step of calculating,successively along the edge detection line, standard deviations ofpicture element concentrations on a straight line which passes throughthe picture element on the edge detection line and extends along thelongitudinal direction of the staggered pattern, and has a length twotimes or more the longitudinal preset pitch between light portions; astep of, based on the magnitude of the calculated picture elementconcentration standard deviation, detecting the picture elementcorresponding to the edge of the sheet material on the edge detectionline; and a step of setting the shape measurement line with the detectedpicture element corresponding to the edge being a reference.

According to the above-described preferable configuration, in thepicture element region in which the staggered pattern on the edgedetection line is present, both of the light portion and the darkportion are always present on the straight line having a length twotimes or more the longitudinal preset pitch between the light portions(hereinafter, as appropriate, referred to as a “standard deviationmeasurement line”). This causes a larger standard deviation of pictureelement concentrations on the standard deviation measurement line. Onthe other hand, in the picture element region in which the staggeredpattern on the edge detection line is absent, the fact that only thedark portion is present causes a smaller standard deviation of pictureelement concentrations on the standard deviation measurement line.Therefore, based on the magnitude of picture element concentrationstandard deviation, the picture element corresponding to the edge of thesheet material on the edge detection line can be detected. If the shapemeasurement line is set with this picture element corresponding to theedge being a reference, even if the meanders and camber are produced onthe sheet material, the shape measurement line can be set correctly at adesired position, for example, in the central portion in the widthdirection of the sheet material. In the case where the sheet materialruns on transfer rolls, by the incidence of light reflected by thetransfer roll into the image pickup device, a light picture elementregion corresponding to the transfer roll may be caused to be present inthe pattern image. At this time, if the edge detection line is set at aposition at which the picture element region corresponding to thetransfer roll is present, there is a fear that the picture elementcorresponding to the sheet material edge cannot be detected properly.Therefore, in the case where the sheet material runs on the transferrolls, it is preferable that the edge detection line be set at aposition at which the picture element region corresponding to thetransfer roll is absent.

When the staggered pattern projected onto the sheet material surface isphotographed by the image pickup device, on account of the illuminanceunevenness of the light source for projecting the pattern, theinclination angle of the sheet material surface, and the like, in somecases, great unevenness is produced in the picture element concentrationof the pattern image acquired by the image pickup device. Specifically,the illuminance unevenness of the light source or the disposition of theimage pickup device at a position at which the specularly reflectedlight reflected by the sheet material surface of the staggered patterncan be received tends to lighten the central portion of staggeredpattern. In the case where the unevenness of picture elementconcentration of the pattern image is large, as in the case where theabove-described conventional linear pattern is used, there arises aproblem that if the sensitivity of the image pickup device is too high,the staggered pattern is liable to collapse in the picture elementregion of the central portion in which the picture element concentrationof pattern image is high, and on the other hand, if the sensitivity ofthe image pickup device is too low, the light portion of the staggeredpattern is difficult to distinguish in the picture element region of theperipheral portion in which the picture element concentration of patternimage is low.

As the countermeasures for avoiding the above-described problem, it isconceivable that two image pickup devices each having differentsensitivity are arranged in parallel so that the image pickup visualfields thereof have portions overlapping with each other, and the shapemeasurement line is set at the corresponding position in the patternimages acquired by the image pickup devices. In the case where the shapemeasurement line passes through the picture element region in which thepicture element concentration is very high, the surface shape of thesheet material has only to be calculated by using the average pictureelement concentration distribution along the shape measurement line setin the pattern image acquired by a low-sensitivity image pickup device.On the other hand, in the case where the shape measurement line passesthrough the picture element region in which the picture elementconcentration is low, the surface shape of the sheet material has onlyto be calculated by using the average picture element concentrationdistribution along the shape measurement line set in the pattern imageacquired by a high-sensitivity image pickup device. Specifically, in theaverage picture element concentration distribution along the shapemeasurement line set in the pattern image acquired by thehigh-sensitivity image pickup device, in the case where the number ofpicture elements in which the concentration saturates is large, thesurface shape of the sheet material has only to be calculated by usingthe average picture element concentration distribution along the shapemeasurement line set in the pattern image acquired by a low-sensitivityimage pickup device. On the other hand, in the average picture elementconcentration distribution along the shape measurement line set in thepattern image acquired by the high-sensitivity image pickup device, inthe case where the number of picture elements in which the concentrationsaturates is small, the surface shape of the sheet material has only tobe calculated by using the average picture element concentrationdistribution along the shape measurement line set in the pattern imageacquired by a high-sensitivity image pickup device.

Therefore, preferably, in the method of the present invention, as theimage pickup device, a high-sensitivity image pickup device and alow-sensitivity image pickup device having a sensitivity lower than thatof the high-sensitivity image pickup device are used; in the secondstep, the high-sensitivity image pickup device and the low-sensitivityimage pickup device are arranged in parallel so that the image pickupvisual fields thereof have portions overlapping with each other; in thethird step, the shape measurement line is set at the correspondingposition in the pattern images acquired by the high-sensitivity imagepickup device and the low-sensitivity image pickup device respectively;and the sixth step comprises: a step of counting the number of pictureelements in which the concentration saturates, in the average pictureelement concentration distribution along the shape measurement line setin the pattern image acquired by the high-sensitivity image pickupdevice, and a step of, if the number of concentration saturated pictureelements is not smaller than a preset predefined threshold value,calculating the surface shape of the sheet material along the shapemeasurement line based on the average picture element concentrationdistribution along the shape measurement line set in the pattern imageacquired by the low-sensitivity image pickup device, and if the numberof concentration saturated picture elements is smaller than the presetthreshold value, calculating the surface shape of the sheet materialalong the shape measurement line based on the average picture elementconcentration distribution along the shape measurement line set in thepattern image acquired by the high-sensitivity image pickup device.

In the case where the sheet material is a hot-rolled steel sheet, insome cases, the local flatness is too poor, a thin steel sheet risesmomentarily, or abnormal scale is formed partially on the steel sheetsurface, and therefore there may arise an accident such that theflatness cannot be measured satisfactorily. The control of the finishingmill or the like using an abnormal measured value leads to wrongcontrol, which results in further deterioration in flatness or a troubleon the production line. It is preferable that even if such an abnormalmeasured value occurs, the control be performed as far as possible, andon the other hand, if abnormalities of measured value occurcontinuously, the control be halted.

On the other hand, if the measurement range in the longitudinaldirection of the hot-rolled steel sheet is about 1 m, the standing wavesof sheet observed in the pattern image have about one to three peaks.Therefore, since the measured flatness value measured by using onepattern image varies, it is preferable that a value obtained byaveraging the measured flatness values of latest several times beoutputted to control the finishing mill and the like.

The measurement response speed necessary for carrying out feedbackcontrol to the finishing mill and the like is about 1 second (because ofthe transfer delay from a flatness measuring instrument to the finishingmill and the like, requirement for a high response speed higher thanthis speed is meaningless). However, by the recent advances in computertechnology, 20 or more of pattern images can be processed for onesecond, so that even if some degree of averaging is performed, theaveraged value can be satisfactorily applied to the control of thefinishing mill and the like.

Therefore, preferably, the method of the present invention furthercomprises: a seventh step of measuring the flatness successively in aplurality of different portions in the longitudinal direction of thesheet material by repeatedly executing the first to sixth steps for thesheet material running in the longitudinal direction; an eighth step ofdetermining whether or not the measured flatness values of preset latestN times (N: integer of 2 or more) succeeded in measurement; and a ninthstep of, if among the measured flatness values of the latest N times,the number of times when it is determined that the measurement issuccessful is not smaller than a preset threshold value, generating asignal indicative of success in measurement, and outputting the averagevalue of the measured flatness value succeeded in measurement among themeasured flatness values of the latest N times as a flatness measurementresult, and if among the measured flatness values of the latest N times,the number of times when it is determined that the measurement issuccessful is smaller than the threshold value, generating a signalindicative of failure in measurement.

According to the above-described preferable configuration, even if theflatness measurement fails momentarily for some reason (in the casewhere the number of times when it is determined that the measurement issuccessful is not smaller than the threshold value), a signal indicativeof success in measurement is generated, and the average value of themeasured flatness value succeeded in measurement is outputted as aflatness measurement result. Therefore, if these outputs are inputted toa control unit for controlling the finishing mill and the like, and thecontrol unit carries out control based on this input, the control on thebasis of the measured flatness value is carried on continuously. Also,if the measurement fails continuously (in the case where the number oftimes when it is determined that the measurement is successful issmaller than the threshold value), a signal indicative of failure inmeasurement is outputted. Therefore, if this output is inputted to thecontrol unit for controlling the finishing mill and the like, thecontrol can be halted suitably. For example, as the number of times Nfor averaging, 10 times can be exemplified, and as the threshold value,five times can be exemplified.

The determination in the eighth step as to whether or not the measuredflatness value succeeded in measurement can be made, for example, byboth determinations as to whether or not the edge of sheet materialcould be detected properly and as to whether or not the surface shape ofsheet material along the shape measurement line could be calculatedproperly. The determination as to whether or not the sheet material edgecould be detected properly can be made, for example, by determiningwhether or not the width and meander amount of sheet material that canbe calculated from the coordinate of picture element corresponding tothe sheet material edge detected in the pattern image take abnormalvalues. Also, the determination as to whether or not the surface shapeof the sheet material along the shape measurement line could becalculated properly can be made, for example, by determining whether ornot the amplitude of the average picture element concentrationdistribution along the shape measurement line is excessively small.

Therefore, preferably, the eighth step in the method of the presentinvention comprises: a step of setting two edge detection linesextending in the transverse direction of the staggered pattern atdifferent positions in the longitudinal direction of the staggeredpattern in each of the pattern images used to obtain the measuredflatness values of the latest N times; a step of detecting the pictureelement corresponding to the edge of the sheet material on each of theedge detection lines; and a step of, based on a coordinate of thedetected picture element corresponding to the edge of the sheet materialand the amplitude of the average picture element concentrationdistribution along the shape measurement line calculated in the fifthstep, determining whether or not the measured flatness values of thelatest N times succeeded in measurement.

The present invention also provides a method for producing a steel sheetin which method a steel sheet is produced by rolling a slab, which isroughly rolled by a roughing mill, by using a finishing mill train, andthereafter by being cooled by a cooling zone, wherein the flatness ofthe steel sheet is measured by the above-described flatness measuringmethod, and based on the measurement result, rolling conditions of thefinishing mill or cooling conditions of the cooling zone are controlled.

According to the present invention, even in the case where an imagepickup device is arranged at a position at which the specularlyreflected light of a light and dark pattern projected onto the surfaceof a sheet material having a strong specular reflection property can bereceived, the surface shape of the sheet material can be measured withhigh accuracy, whereby the flatness of the sheet material can bemeasured with high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1]

FIG. 1 is a schematic view showing a configuration example of a devicefor carrying out the grating pattern projection method.

[FIG. 2]

FIG. 2 is an explanatory view for explaining the range in which a camerareceives specularly reflected light of a projector projecting light.

[FIG. 3]

FIG. 3 is an explanatory view for explaining various light and darkpatterns by comparison.

[FIG. 4]

FIG. 4 is a schematic view showing the relationship between thelongitudinal pitch p_(m) between light portions of the staggered patternand the inclination angle θ of the sheet material surface.

[FIG. 5]

FIG. 5 is a schematic view showing an outline configuration example of aflatness measuring apparatus for carrying out the flatness measuringmethod in accordance with the present invention.

[FIG. 6]

FIG. 6 is a schematic view showing the installation state of theflatness measuring apparatus shown in FIG. 5.

[FIG. 7]

FIG. 7 is a graph showing the relationship between p_(m)/p_(s) and theinclination angle θ of the surface of the hot-rolled steel sheet underthe arrangement condition according to one embodiment of the presentinvention.

[FIG. 8]

FIG. 8 (FIGS. 8A and 8B) is a plan view showing one example of staggeredpattern formed on the slide composing the projector shown in FIG. 5.

[FIG. 9]

FIG. 9 is a plan view showing another example of staggered patternformed on the slide composing the projector shown in FIG. 5.

[FIG. 10]

FIG. 10 is a flow chart of processing performed by the image analyzingdevice shown in FIG. 5.

[FIG. 11]

FIG. 11 is an explanatory view for explaining a method for detecting theedge of the hot-rolled steel sheet and a method for determining theshape measurement line.

[FIG. 12]

FIG. 12 is an explanatory view for explaining the method for calculatingthe degree of steepness.

[FIG. 13]

FIG. 13 is views showing pattern image examples in the case where theconventional linear pattern is used as a light and dark patternprojected onto the surface of the hot-rolled steel sheet and pictureelement concentration distributions in the pattern image along the shapemeasurement line in the central portion in the width direction of thehot-rolled steel sheet and along the shape measurement line in thevicinity of the right-hand side edge thereof.

[FIG. 14]

FIG. 14 is views showing pattern image examples in the case where thestaggered pattern is used as a light and dark pattern projected onto thesurface of the hot-rolled steel sheet and average picture elementconcentration distributions in the pattern image along the shapemeasurement line in the central portion in the width direction of thehot-rolled steel sheet and along the shape measurement line in thevicinity of the right-hand side edge thereof.

[FIG. 15]

FIG. 15 (FIGS. 15A, 15B, 15C and 15D) shows measurement examples ofdegree of steepness and the like over the overall length of one coil ofsteel sheet in the case where the conventional linear pattern is used asa light and dark pattern projected onto the surface of the hot-rolledsteel sheet.

[FIG. 16]

FIG. 16 (FIGS. 16A, 16B, 16C and 16D) shows measurement examples ofdegree of steepness and the like over the overall length of one coil ofsteel sheet in the case where the staggered pattern is used as a lightand dark pattern projected onto the surface of the hot-rolled steelsheet S.

[FIG. 17]

FIG. 17 (FIGS. 17A, 17B, 17C and 17D) shows measurement examples ofdegree of steepness and the like over the overall length of one coil ofsteel sheet in the case where the staggered pattern is used as a lightand dark pattern projected onto the surface of the hot-rolled steelsheet for the hot-rolled steel sheet having material quality of lowsurface reflectance.

DESCRIPTION OF EMBODIMENTS

One embodiment of the present invention will now be described withreference to the accompanying drawings as appropriate. In thedescription below, explanation is given by taking as an example the casewhere the sheet material is a hot-rolled steel sheet, and the flatness(degree of steepness) thereof is measured on the exit of a finishingmill train on a hot-rolled steel sheet production line.

<1. General Configuration of Flatness Measuring Apparatus>

FIG. 5 is a schematic view showing an outline configuration example of aflatness measuring apparatus for carrying out the flatness measuringmethod in accordance with the present invention. FIG. 6 is a schematicview showing the installation state of the flatness measuring apparatusshown in FIG. 5. As shown in FIGS. 5 and 6, a flatness measuringapparatus 100 of this embodiment includes a projector 1 for projecting astaggered pattern P, which is a light and dark pattern, onto the surfaceof a hot-rolled steel sheet S running horizontally in the longitudinaldirection so that the longitudinal direction of the staggered pattern Pcorresponds to the longitudinal direction of the hot-rolled steel sheetS and the transverse direction of the staggered pattern P corresponds tothe width direction of the hot-rolled steel sheet S; an image pickupdevice 2 that has an image pickup visual field larger than the width ofthe hot-rolled steel sheet S, and acquires a pattern image byphotographing the staggered pattern P projected onto the surface of thehot-rolled steel sheet S; and an image analyzing device 3 for analyzingthe pattern image acquired by the image pickup device 2.

As shown in FIG. 6, the installation space on the exit of the finishingmill train in which the flatness measuring apparatus 100 of thisembodiment is installed merely has a length of 2 m in the longitudinaldirection of the hot-rolled steel sheet S and a height of 2.5 m in thevertical direction. Therefore, in order to secure at least 1 m ofmeasurement range (image pickup visual field) in the longitudinaldirection of the hot-rolled steel sheet S, the image pickup device 2must be arranged at a position at which the specularly reflected lightprojected from the projector 1 (the specularly reflected light of thestaggered pattern P) can be received. In this embodiment, the staggeredpattern P is projected onto the hot-rolled steel sheet S at an angle of15° (the angle between the vertical direction and the projectiondirection of the staggered pattern P) from the slantwise upside withrespect to the hot-rolled steel sheet S, and this projected staggeredpattern P is photographed at an angle of 15° (the angle between thevertical direction and the image pickup direction) from the slantwiseupside with respect to the hot-rolled steel sheet S by the image pickupdevice 2.

FIG. 7 is a graph showing the relationship between p_(m)/p_(s) and theinclination angle θ of the surface of the hot-rolled steel sheet S underthe above-described arrangement condition. As described above, p_(m)denotes a longitudinal pitch between light portions of the staggeredpattern P in the pattern image acquired for the hot-rolled steel sheetS, p_(s) denotes a longitudinal pitch between light portions of thestaggered pattern P in the pattern image acquired for a referencematerial that is placed horizontally and has a flat surface shape, and θdenotes an inclination angle between the horizontal direction and thesurface of the hot-rolled steel sheet S. The measurement range ofinclination angle θ of the surface of the hot-rolled steel sheet S isdetermined by the sum of the required flatness (degree of steepness)measurement range and the range of inclination angle of the whole of thehot-rolled steel sheet S that may be formed at the measurement time. Inthis embodiment, since the required measurement range of degree ofsteepness is −5% to +5% (corresponding to −9° to +9° if converted intothe inclination angle of the surface of the hot-rolled steel sheet S),and additionally considering the allowance of a change in inclinationangle of the whole of the hot-rolled steel sheet S caused by thefluttering of the hot-rolled steel sheet S, the measurement range ofinclination angle θ of the surface of the hot-rolled steel sheet S ismade −15° to +15°. FIG. 7 reveals that if the inclination angle of thesurface of the hot-rolled steel sheet S changes in the range of −15° to+15°, p_(m)/p_(s) changes in the range of 0.85 to 1.15.

<2. Configuration of Projector>

In this embodiment, as a light source composing the projector 1, a metalhalide lamp having a rated power of 2.5 kW is used. The light emittedfrom this lamp passes through a slide and an imaging lens arranged infront of the lamp and is projected onto the surface of the hot-rolledsteel sheet S at an imaging magnification of about ×20. The distancefrom the projector 1 to the surface of the hot-rolled steel sheet S isabout 2 m, and the dimensions of the projected staggered pattern are1400 mm in the longitudinal direction and 1800 mm in the transversedirection. On the slide, the staggered pattern is formed by depositingCr on a quartz glass substrate. The portion on which Cr is deposited isthe dark portion of the staggered pattern, and the portion on which Cris not deposited is the light portion of the staggered pattern.

FIG. 8 (FIGS. 8A and 8B) is a plan view showing one example of staggeredpattern formed on the slide composing the projector, FIG. 8A being ageneral view, and FIG. 8B being a partial enlarged view. As shown inFIG. 8, on the slide, light portions M are arranged in a staggered format a pitch of 2 mm in the longitudinal direction and the transversedirection respectively. As described above, the imaging magnification isabout ×20, so that onto the surface of the hot-rolled steel sheet S isprojected the staggered pattern P in which the light portions M arearranged in a staggered form at a 40-mm pitch in the longitudinaldirection and the transverse direction respectively (that is, thelongitudinal preset pitch P_(L)=40 mm, the transverse preset pitchP_(W)=40 mm).

The staggered pattern is not limited to the pattern shown in FIG. 8, andcan be made a staggered pattern in which, for example, one light portionM is divided into two elements MA and MB without changing the pitch ofthe light portion M as shown in FIG. 9. Also, the influence ofilluminance unevenness of the projector 1 can be restrained by devisingso that the size of the light portion M is changed partially. In thisembodiment, the illuminance in the vicinity of the surface of thehot-rolled steel sheet S is about 6000 Lx in the vicinity of the opticalaxis of the projector 1 and about 3000 Lx in the vicinity of the edge ofthe hot-rolled steel sheet M. In this embodiment, the whole of theprojector 1 is housed in a dustproof box made of a stainless steelbecause the projector 1 is installed at the site at which dust particlesand foggy waterdrops scatter in large amounts. Also, the projector 1 hasa construction such that air is supplied into the dustproof box by usinga large-sized blower and is let to blow off to the outside through anopening for projecting the staggered pattern to prevent dust particlesand foggy waterdrops from intruding into the dustproof box through theopening

<3. Configuration of Image Pickup Device>

In this embodiment, as the image pickup device 2, a two-dimensional CCDcamera is used which has a SVGA-size image sensor (the image sensor has788 light-receiving elements in the transverse direction, 580light-receiving elements in the longitudinal direction) and outputs 40image signals per second in a progressive system. This CCD camera isconfigured so that a plurality of cameras can pick up imagessynchronously by means of a synchronization signal sent from theoutside. In this embodiment, as the image pickup device 2, two CCDcameras 21 and 22 described above are used. The CCD cameras 21 and 22are arranged in parallel so that the image pickup visual fields thereofhave portions overlapping with each other, and by the adjustment of lensstop and gain of respective cameras, the sensitivity is set at 1:4(hereinafter, as appropriate, the CCD camera having lower sensitivity isreferred to as the low-sensitivity image pickup device 21, and the CCDcamera having higher sensitivity is referred to as the high-sensitivityimage pickup device 22).

In this embodiment, the exposure time of the image pickup device 2 isset at 2.5 msec so that the surface shape of the hot-rolled steel sheetS that is rolled and coiled at a high speed of 1500 mpm at the maximumcan be measured without blurring. Also, the image pickup device 2 ofthis embodiment is provided with a band-pass filter allowing onlyblue-green color to penetrate in front of the lens so that the staggeredpattern can be photographed clearly without being affected by theradiation light radiated from the surface of the hot-rolled steel sheetS even when the hot-rolled steel sheet S having a temperature of 950° C.is measured. The image pickup device 2 of this embodiment as well is,like the projector 1, housed in a dustproof box made of a stainlesssteel, and is air purged by pressurized air to prevent the lens frombeing stained. The image pickup device 2 of this embodiment has an imagepickup visual field of about 1800 mm in the width direction of thehot-rolled steel sheet S, so that the transverse resolution of thepattern image acquired by the image pickup device 2 is about 2.3 mm perpicture element.

<4. Configuration of Image Analyzing Device>

The image analyzing device 3 of this embodiment has a configuration suchthat a program for executing the later-described processing(hereinafter, referred to as a flatness analyzing program) is installedin a general-purpose personal computer (CPU: Core2Duo processor of2.4-GHz clock frequency, OS: Windows (registered trademark)). The imageanalyzing device 3 is configured so as to capture image signalsoutputted from the low-sensitivity image pickup device 21 and thehigh-sensitivity image pickup device 22 in a memory simultaneously at256 gradation (8 bits) by using an incorporated multichannel imagecapturing board. The image data (pattern image) captured in the memoryof the image analyzing device 3 is analyzed by the flatness analyzingprogram, and the measured flatness value as an analysis result isoutputted to the monitor screen of the image analyzing device 3 and ahost control unit (a control unit for controlling the finishing mill andthe like).

<5. Processing Details of Flatness Analyzing Program>

The image analyzing device 3 performs processing on the pattern imagephotographed and acquired by the image pickup device 2 by the procedureshown in FIG. 10 by using the installed flatness analyzing program.Hereunder, the processing is explained successively.

<5-1. Processing for Detection of Edge of Hot-Rolled Steel Sheet and forSetting of Shape Measurement Lines (S1 in FIG. 10)>

FIG. 11 is an explanatory view for explaining a method for detecting theedge of the hot-rolled steel sheet and a method for determining theshape measurement line. In detecting the edge of the hot-rolled steelsheet S, first, edge detection lines LE1 and LE2 extending in thetransverse direction of the staggered pattern P at predeterminedpositions (two different positions in the longitudinal direction of thestaggered pattern P) are set in the pattern image acquired by thehigh-sensitivity image pickup device 22.

Next, the standard deviations of picture element concentrations on astraight line that passes through the picture element on the edgedetection line LE1 and extends along the longitudinal direction of thestaggered pattern P, and has a length (100 mm in this embodiment) twotimes or more the longitudinal preset pitch (in this embodiment, thelongitudinal preset pitch P_(L)=40 mm) between the light portions arecalculated successively along the edge detection line LE1. Based on themagnitude of the calculated picture element concentration standarddeviation, picture elements E11 and E12 corresponding to the edge of thehot-rolled steel sheet S are detected on the edge detection line LE1.Specifically, for example, the distribution of the picture elementconcentration standard deviations along the edge detection line LE1 hasonly to be differentiated along the edge detection line LE1, and thepicture element E11 in which the differentiated intensity is at themaximum and the picture element E11 in which the differentiatedintensity is at the minimum have only to be detected as picture elementscorresponding to the edge of the hot-rolled steel sheet S. For the edgedetection line LE2 as well, likewise, based on the magnitude of thecalculated picture element concentration standard deviation calculatedsuccessively along the edge detection line LE2, picture elements E21 andE22 corresponding to the edge of the hot-rolled steel sheet S aredetected on the edge detection line LE2. By the above-describedprocessing, the straight line passing through the picture elements E11and E21 and the straight line passing through the picture elements E12and E22 are detected as estimated edges LL and LR of the steel sheet S,respectively.

Based on the coordinates of the detected picture elements E11 and E12and the transverse resolution of pattern image (in this embodiment,about 2.3 mm per picture element), the width of the hot-rolled steelsheet S on the edge detection line LE1 can be calculated. Likewise,based on the coordinates of the detected picture elements E21 and E22and the transverse resolution of pattern image, the width of thehot-rolled steel sheet S on the edge detection line LE2 can becalculated. In the case where the difference between the width of thehot-rolled steel sheet S on the edge detection line LE1 and the width ofthe hot-rolled steel sheet S on the edge detection line LE2 is large(for example, 10 mm or larger), it can be determined that the edge ofthe hot-rolled steel sheet S could not be detected properly. Also, fromthe coordinates of the detected picture elements E11 and E12, thecoordinate of the central portion of the hot-rolled steel sheet S on theedge detection line LE1 can be calculated. Likewise, the coordinate ofthe central portion of the hot-rolled steel sheet S on the edgedetection line LE2 can be calculated from the coordinates of thedetected picture elements E21 and E22. Based on the difference betweenthe coordinate of the central portion of the hot-rolled steel sheet S onthe edge detection line LE1 and the coordinate of the central portion ofthe hot-rolled steel sheet S on the edge detection line LE2 and thetransverse resolution of pattern image, the meander amount of thehot-rolled steel sheet S can be calculated. If this meander amount islarger than a predefined threshold value, it can be determined that theedge of the hot-rolled steel sheet S could not be detected properly.

The shape measurement lines are determined with the picture elements E11to E22 corresponding to the edge detected as described above beingreferences (with a left-hand side estimated edge LL passing through thepicture elements E11 and E21 and a right-hand side estimated edge LRpassing through the picture elements E12 and E22 being references), andare set in the pattern image acquired by the high-sensitivity imagepickup device 22. Specifically, in this embodiment, there are set atotal of five shape measurement lines: a shape measurement line L11 inthe vicinity of the left-hand side edge of the hot-rolled steel sheet S(on the inside by 75 mm from the left-hand side estimated edge LL), ashape measurement line L12 on the inside by a length corresponding toone-fourth of the width of the hot-rolled steel sheet S from theleft-hand side edge of the hot-rolled steel sheet S (on the inside by alength corresponding to one-fourth of the width of the hot-rolled steelsheet S from the left-hand side estimated edge LL), a shape measurementline L13 in the central portion in the width direction of the hot-rolledsteel sheet S, a shape measurement line L14 on the inside by a lengthcorresponding to one-fourth of the width of the hot-rolled steel sheet Sfrom the right-hand side edge of the hot-rolled steel sheet S (on theinside by a length corresponding to one-fourth of the width of thehot-rolled steel sheet S from the right-hand side estimated edge LR),and a shape measurement line L15 in the vicinity of the right-hand sideedge of the hot-rolled steel sheet S (on the inside by 75 mm from theright-hand side estimated edge LR).

The positional relationship between the coordinates in the pattern imageacquired by the high-sensitivity image pickup device 22 and thecoordinates in the pattern image acquired by the correspondinglow-sensitivity image pickup device 21 is determined in advance.Thereby, for the pattern image acquired by the low-sensitivity imagepickup device 21, the shape measurement lines can be set at positionscorresponding to the shape measurement lines L11 to L15 set for thepattern image acquired by the high-sensitivity image pickup device 22.

<5-2. Processing for Calculation of Average Picture ElementConcentration Distribution Along Shape Measurement Line (S2 in FIG. 10)>

In this processing, in the pattern image acquired by each of thelow-sensitivity image pickup device 21 and the high-sensitivity imagepickup device 22, the picture element concentrations on a straight linethat passes through the picture elements on the shape measurement linesL11 to L15 and extends in the transverse direction of the staggeredpattern, and has a length two times or more the transverse preset pitchbetween the light portions (in this embodiment, the transverse presetpitch P_(W)=40 mm) are averaged, whereby the average picture elementconcentration is calculated. In this embodiment, since the transverseresolution of the pattern image is about 2.3 mm per picture element asdescribed above, the length of the straight line on which the pictureelement concentrations are averaged has only to be 35 picture elementsor more. Therefore, in this embodiment, the length of the straight lineon which the picture element concentrations are averaged is made 40picture elements, and thereby the distribution of the average pictureelement concentrations along each of the shape measurement lines L11 toL15 is calculated. Also, the x coordinate (the position along thelongitudinal direction of the staggered pattern in the pattern image) oneach of the shape measurement lines L11 to L15 calculates the averagepicture element concentration distribution in the range of 50 to 561 inpicture element units (that is, 512 pieces of average picture elementdata).

<5-3. Processing for Determination of Respective Use of Low-SensitivityImage Pickup Device and High-Sensitivity Image Pickup Device (S3 in FIG.10)>

In this processing, in the average picture element concentrationdistribution along each of the shape measurement lines L11 to L15 set inthe pattern image acquired by the high-sensitivity image pickup device22, the number of picture elements in which the concentration saturatesis counted. Specifically, in this embodiment, if the concentrationexceeds 250, it is thought that the concentration saturates, and thenumber of picture elements (the number of concentration saturatedpicture elements) is counted. As the result, if the number ofconcentration saturated picture elements is not smaller than a presetpredefined threshold value, the average picture element concentrationdistribution along the shape measurement line set in the pattern imageacquired by the low-sensitivity image pickup device 21 is used (asdescribed later, the surface shape of the hot-rolled steel sheet S alongthe shape measurement line is calculated by using this average pictureelement concentration distribution). On the other hand, if the number ofconcentration saturated picture elements is smaller than the presetthreshold value, the average picture element concentration distributionalong the shape measurement line set in the pattern image acquired bythe high-sensitivity image pickup device 22 is used. Specifically, forexample, in the average picture element concentration distribution alongthe shape measurement line L11 set in the pattern image acquired by thehigh-sensitivity image pickup device 22, if the number of concentrationsaturated picture elements is not smaller than the threshold value, theaverage picture element concentration distribution along the shapemeasurement line L11 set in the pattern image acquired by thelow-sensitivity image pickup device 21 is used. Also, for example, inthe average picture element concentration distribution along the shapemeasurement line L13 set in the pattern image acquired by thehigh-sensitivity image pickup device 22, if the number of concentrationsaturated picture elements is smaller than the threshold value, theaverage picture element concentration distribution along the shapemeasurement line L13 set in the pattern image acquired by thelow-sensitivity image pickup device 21 is used.

<5-4. Processing for Calculation of Inclination Angle Distribution ofHot-Rolled Steel Sheet Along Shape Measurement Line and Surface Shape(S4 in FIG. 10)>

In this processing, based on the average picture element concentrationdistribution along the shape measurement lines L11 to L15 calculated asdescribed above for the hot-rolled steel sheet S, on which flatness ismeasured, the distribution p_(m)(x) of longitudinal pitch between lightportions of the staggered pattern P along the shape measurement linesL11 to L15 is calculated.

On the other hand, for the reference material that is placedhorizontally and has a flat surface shape as well, the same processingas described above is performed, and the average picture elementconcentration distribution along the shape measurement lines L11 to L15in the pattern image acquired on the reference material is calculated.Based on the average picture element concentration distribution alongthe shape measurement lines L11 to L15, the distribution p_(s)(x) oflongitudinal pitch between light portions of the staggered pattern alongthe shape measurement lines L11 to L15 is calculated in advance.

As a method for calculating the distributions p_(m)(x) and p_(s)(x) oflongitudinal pitch between light portions based on the average pictureelement concentration distribution, various methods are conceivable. Inthis embodiment, the phase analysis method explained below is applied.

Hereunder, the phase analysis method applied to the above-describedaverage picture element concentration distribution will be explained.

The average picture element concentration distribution obtained for thehot-rolled steel sheet S, on which flatness is measured, is taken asf(x). If, by applying the frequency analysis method such as the Fouriertransform method to this f(x), only a space frequency componentcorresponding to the assumed change width (for example, −5% to +5%) ofthe longitudinal pitch between light portions of the staggered patternis drawn out of f(x), the distribution f_(s)(x) represented by Formula(9) is obtained. Since this f_(s)(x) includes only the distribution ofthe longitudinal pitches between light portions of the projectedstaggered pattern as a periodic component, the distribution of thelongitudinal pitches can be determined by analyzing a phase componentφ(x).f _(S)(x)=A(x)sin φ(x)  (9)

For the analysis of phase component, for example, Hilbert transformationcan be used. The Hilbert transformation is transformation into awaveform of the same amplitude, in which the phase shifts by π/2)(90°with respect to the original waveform. In the calculation method foraccomplishing the Hilbert transformation, the coefficient of negativefrequency portion of F_(s)(k) obtained by subjecting f_(s)(x) todiscrete Fourier transform is replaced with 0, and the fact that theresult of discrete reverse Fourier transform is f_(S)(x)+if_(H)(x) isutilized. Since the phase of the obtained f_(H)(x) shifts by π/2 withrespect to f_(S)(x), f_(S)(x) is represented by Formula (10).

$\begin{matrix}{{f_{H}(x)} = {{{A(x)}\sin\left\{ {{\phi(x)} - \frac{\pi}{2}} \right\}} = {{- {A(x)}}\cos\;{\phi(x)}}}} & (10)\end{matrix}$

Therefore, the result of calculation of arc tangent (inverse tangentfunction) of f_(S)(x)/f_(H)(x) is equal to −φ(x), which is a phasecomponent, as shown in Formula (11).

$\begin{matrix}{{\tan^{- 1}\left\{ \frac{f_{S}(x)}{f_{H}(x)} \right\}} = {{{- \tan^{- 1}}\left\{ \frac{{A(x)}\sin\;{\phi(x)}}{{A(x)}\cos\;{\phi(x)}} \right\}} = {- {\phi(x)}}}} & (11)\end{matrix}$

Since the obtained φ(x) has been wrapped (folded each π), π is added orsubtracted at each folding point (unwrapping processing) to produce acontinuous waveform. Also, as shown in Formula (12), the square sumsquare roots of f_(S)(x) and f_(H)(x) are calculated, and thereby theamplitude A(x) of f_(S)(x) can be determined.√{square root over ({f _(S)(x)}² +{f _(H)(x)}²)}{square root over ({f_(S)(x)}² +{f _(H)(x)}²)}=√{square root over ({A(x)sin(φ(x))}²+{A(x)cos(φ(x))}²)}{square root over ({A(x)sin(φ(x))}²+{A(x)cos(φ(x))}²)}{square root over ({A(x)sin(φ(x))}²+{A(x)cos(φ(x))}²)}{square root over ({A(x)sin(φ(x))}²+{A(x)cos(φ(x))}²)}=A(x)  (12)

Herein, since dφ(x)/dx, which is the differential of phase componentφ(x), is equal to a value obtained by multiplying the space frequencydistribution by 2π, the longitudinal pitch p_(m)(x) between lightportions of the staggered pattern can be determined by Formula (13).

$\begin{matrix}{{p_{m}(x)} = {2{\pi\left( \frac{\mathbb{d}{\phi(x)}}{\mathbb{d}x} \right)}^{- 1}}} & (13)\end{matrix}$

For the average picture element concentration distribution obtained forthe reference material that is placed horizontally and has a flatsurface shape, as well, the same analysis as described above isperformed, whereby the longitudinal pitch p_(s)(x) between lightportions of the staggered pattern can be determined.

Next, in this processing, based on the distributions p_(m)(x) andp_(s)(x) of longitudinal pitches between light portions of the staggeredpattern, which are calculated as described above and Formula (1), thedistribution θ(x) of inclination angles of the surface of the hot-rolledsteel sheet S along the shape measurement lines L11 to L15 iscalculated.

$\begin{matrix}{{\theta(x)} = {\tan^{- 1}\left\{ \frac{\left( {{p_{m}(x)}/{p_{S}(x)}} \right) - 1}{{\tan\;\alpha} + {\left( {{p_{m}(x)}/{p_{S}(x)}} \right)\tan\;\beta}} \right\}}} & (1)\end{matrix}$

In Formula (1), x denotes a position along the longitudinal direction ofthe staggered pattern in the pattern image (a position along thelongitudinal direction of the sheet material), θ(x) denotes thedistribution of inclination angles between the horizontal direction andthe surface of the sheet material, α denotes the angle between thevertical direction and the image pickup direction of the image pickupdevice (15° in this embodiment), and β denotes the angle between thevertical direction and the projection direction of the staggered pattern(15° in this embodiment).

Finally, in this processing, the inclination angles of the surface ofthe hot-rolled steel sheet S along each of the shape measurement linesL11 to L15, which are calculated as described above, are integratedalong each of the shape measurement lines L11 to L15, whereby thesurface shape of the hot-rolled steel sheet S along each of the shapemeasurement lines L11 to L15 can be calculated.

The determination as to whether or not the surface shape of thehot-rolled steel sheet S along each of the shape measurement lines L11to L15 could be calculated properly can be made, for example, bydetermining whether or not the amplitude of the average picture elementconcentration distribution along each of the shape measurement lines L11to L15 is excessively small. Specifically, the number of pictureelements in which the amplitude is smaller than the preset thresholdvalue among the amplitudes A(x) calculated by Formula (12) by theabove-described phase analysis of the average picture elementconcentration distribution f(x) is counted. If the number of pictureelements is smaller than a predetermined number, it is determined thatthe surface shape of the hot-rolled steel sheet S could not becalculated properly. If the number of picture elements is not smallerthan the predetermined number, it can be determined that the surfaceshape of the hot-rolled steel sheet S could be calculated properly.

<5-5. Processing for Calculation of Flatness (Degree of Steepness) (S5in FIG. 10)>

In this processing, based on the surface shape of the hot-rolled steelsheet S along each of the shape measurement lines L11 to L15, which iscalculated as described above, the degree of steepness is calculated. Incalculating the degree of steepness, first, based on the surface lengthin a certain section to be measured along each of the shape measurementlines L11 to L15 and the direct distance therebetween, the elongationpercentage on each of the shape measurement lines L11 to L15 iscalculated. Then, the difference in elongation percentage Δε, which is adifference between the elongation percentage ε_(CENT) on the shapemeasurement line L13 in the central portion in the width direction ofthe hot-rolled steel sheet S and the elongation percentage ε_(EDGE) onother shape measurement lines L11, L12, L14 and L15, is calculated(refer to Formula (2)). Then, based on this difference in elongationpercentage Δε and Formula (3), the degree of steepness λ is calculated.

Hereunder, the case where the degree of steepness is determined based onthe surface shape along the shape measurement line L11 in the vicinityof the left-hand side edge and the shape measurement line L13 in thecentral portion in the width direction is explained specifically withreference to FIG. 12.

FIG. 12 is an explanatory view for explaining the method for calculatingthe degree of steepness. The elongation percentage ε_(EDGE) on the shapemeasurement line L11 is calculated by the formula in FIG. 12 based onthe surface length in a section in which a surface shape S11 of thehot-rolled steel sheet S is measured along the shape measurement lineL11 and the direct distance therebetween. Likewise, the elongationpercentage ε_(CENT) on the shape measurement line L13 is calculated bythe formula in FIG. 12 based on the surface length in a section in whicha surface shape S13 of the hot-rolled steel sheet S is measured alongthe shape measurement line L13 and the direct distance therebetween. Inthe example shown in FIG. 12, to restrain the influence of minutemeasurement noise, the section to be measured is divided into 12subsections by points P₀ to P₁₂, and the surface lengths of the surfaceshapes S11 and S13 are calculated by polygonal line approximation. Thedifference in elongation percentage Δε, which is the difference betweenthe elongation percentage ε_(CENT) on the shape measurement line L13 andthe elongation percentage ε_(EDGE) on the shape measurement line L11, iscalculated, and based on this difference in elongation percentage Δε andFormula (3), the degree of steepness λ is calculated.

<5-6. Processing for Determination of Effectiveness of MeasurementResult (S6 in FIG. 10)>

In this processing, the flatness (degree of steepness) is measuredsuccessively in a plurality of different portions in the longitudinaldirection of the hot-rolled steel sheet S as described above, and it isdetermined whether or not the measured flatness values of preset latestN times (N: integer of 2 or more) succeeded in measurement. In thisembodiment, the determination as to whether or not the measured flatnessvalues succeeded in measurement is made by both determinations as towhether or not the edge of the hot-rolled steel sheet S could bedetected properly and as to whether or not the surface shape of thehot-rolled steel sheet S along the shape measurement line could becalculated properly. That is, only when the edge of the hot-rolled steelsheet S could be detected properly and the surface shape of thehot-rolled steel sheet S along the shape measurement line could becalculated properly, it is determined that the measured flatness valuessucceeded in measurement. As described above, the determination as towhether or not the edge of the hot-rolled steel sheet S could bedetected properly is made by determining whether or not the differencebetween the width of the hot-rolled steel sheet S on the edge detectionline LE1 and the width of the hot-rolled steel sheet S on the edgedetection line LE2 is large and whether or not the meander amount of thehot-rolled steel sheet S is larger than the predefined threshold value.Also, for the determination as to the surface shape of the hot-rolledsteel sheet S along the shape measurement line could be calculatedproperly, as described above, the number of picture elements in whichthe amplitude is smaller than the preset threshold value among theamplitudes A(x) calculated by Formula (12) is counted, if the number ofpicture elements is smaller than the predetermined number, it isdetermined that the surface shape of the hot-rolled steel sheet S couldnot be calculated properly, and if the number of picture elements is notsmaller than the predetermined number, it is determined that the surfaceshape of the hot-rolled steel sheet S could be calculated properly.

Next, in this processing, if among the measured flatness values of thelatest N times, the number of times when it is determined that themeasurement is successful is not smaller than a preset threshold valueM, a signal indicative of success in measurement (a signal indicativethat the measurement result is effective) is generated to the controlunit for controlling the finishing mill and the like, and among themeasured flatness values of the latest N times, the average value of themeasured flatness values succeeded in measurement is outputted to thecontrol unit as a flatness measurement result. On the other hand, if thenumber of times when it is determined that the measurement is successfulis smaller than the threshold value M, a signal indicative of failure inmeasurement (a signal indicative that the measurement result isineffective) is generated to the control unit.

In this embodiment, N is set at 10. According to the image analyzingdevice 3 of this embodiment, since 20 pattern images can be processedfor one second, N=10 corresponds to 0.5 second. This is a measurementresponse speed sufficient to use the measured flatness value in thefeedback control to the finishing mill and the like. Also, in thisembodiment, the threshold value M is set at 5. In order to calculate anexact degree of steepness, it is thought that measured values over alength of 5 m three times or more the width (1.65 m at the maximum) ofthe hot-rolled steel sheet S are necessary. Therefore, in thisembodiment, the threshold value M is set at 5 so that the measurementvalues obtained by measuring the range of image visual view of 1 m inthe longitudinal direction of the hot-rolled steel sheet S properly atleast five times are outputted to the control unit.

In this embodiment, the case where the flatness is measured on the exitof the finishing mill train on the hot-rolled steel sheet productionline has been described as an example. However, the present invention isnot limited to this case, and can be applied to the case where theflatness is measured between the finishing mills or on the exit of thecooling zone. Hereunder, there are explained the effects achieved in thecase where the flatness measuring method in accordance with thisembodiment is applied.

<Concerning Picture Element Concentration Distribution>

FIG. 13 is views showing pattern image examples in the case where theconventional linear pattern is used as a light and dark patternprojected onto the surface of the hot-rolled steel sheet S and pictureelement concentration distributions in the pattern image along the shapemeasurement line L13 in the central portion in the width direction ofthe hot-rolled steel sheet S and along the shape measurement line L15 inthe vicinity of the right-hand side edge thereof. FIG. 14 is viewsshowing pattern image examples in the case where the staggered patternof this embodiment is used as a light and dark pattern projected ontothe surface of the hot-rolled steel sheet S and average picture elementconcentration distributions in the pattern image along the shapemeasurement line L13 in the central portion in the width direction ofthe hot-rolled steel sheet S and along the shape measurement line L15 inthe vicinity of the right-hand side edge thereof.

The exposure time of the image pickup device at the time when theconventional linear pattern is photographed is set at 1.5 msec, whereasthe exposure time thereof at the time when the staggered pattern of thisembodiment is photographed is set at 2.5 msec, which is longish time, asdescribed above because the staggered pattern is difficult to collapseeven if the picture element concentration saturates. The hot-rolledsteel sheets S to be measured have the same material quality and thesame size, and are sheets in which poor flatness is produced in thevicinity of the front end.

As can be seen in FIGS. 13 and 14, even if either the linear pattern(FIG. 13) or the staggered pattern (FIG. 14) is used as a light and darkpattern, the influence of specularly reflected light decreases at aposition farther from the central portion of the hot-rolled steel sheetS. Therefore, for the picture element concentration distribution alongthe shape measurement line L15 in the vicinity of the edge (in the caseof FIG. 14, the average picture element concentration distribution),periodic waveform can be observed over the whole region in thelongitudinal direction of pattern image by using the pattern imageacquired by the high-sensitivity image pickup device.

On the other hand, in the central portion of the hot-rolled steel sheetS, in the case where the conventional linear pattern is projected, thedifference in picture element concentration between the picture elementregion corresponding to the position at which specularly reflected lightis received and other picture element regions is large. Therefore, asshown in FIG. 13, for the picture element concentration distributionalong the shape measurement line L13 in the central portion in the widthdirection of the hot-rolled steel sheet S, periodic waveform can beobserved over the whole region in the longitudinal direction of patternimage even by using the pattern image acquired by either of thehigh-sensitivity image pickup device and the low-sensitivity imagepickup device. On the other hand, in the case where the staggeredpattern of this embodiment is projected, as shown in FIG. 14, thestaggered pattern is difficult to collapse even if the picture elementconcentration saturates, and the picture element concentrations areaveraged in the width direction, so that the difference in pictureelement concentration between the picture element region correspondingto the position at which specularly reflected light is received andother picture element regions is small. Therefore, for the pattern imageacquired by the low-sensitivity image pickup device, in the averagepicture element concentration distribution along the shape measurementline L13 in the central portion in the width direction of the hot-rolledsteel sheet S, periodic waveform can be observed over almost the wholeregion in the longitudinal direction of pattern image.

<Measurement Chart of Degree of Steepness etc.>

FIG. 15 shows measurement examples of degree of steepness and the likeover the overall length of one coil of steel sheet in the case where theconventional linear pattern is used as a light and dark patternprojected onto the surface of the hot-rolled steel sheet S. FIG. 16shows measurement examples of degree of steepness and the like over theoverall length of one coil of steel sheet in the case where thestaggered pattern of this embodiment is used as a light and dark patternprojected onto the surface of the hot-rolled steel sheet S. FIGS. 15Aand 16A show the measured values of degree of steepness measured alongthe shape measurement lines L11 and L15 in the vicinity of both theedges, FIGS. 15B and 16B show the number of times when the measurementwas successful among the measured flatness values of the latest tentimes, FIGS. 15C and 16C show whether or not the edge of the hot-rolledsteel sheet S could be detected, and FIGS. 15D and 16D show the numberof shape measurement lines along which the surface shape could bemeasured properly. The hot-rolled steel sheets S to be measured have thesame material quality and the same size, and are sheets in which poorflatness is produced in the vicinity of the front end.

As shown in FIG. 15, in the case where the linear pattern is used as alight and dark pattern, the edge detection is made properly over theoverall length of the hot-rolled steel sheet S (FIG. 15C); however, themeasurement of surface shape cannot be made properly along all of thefive shape measurement lines, and in some cases, the measurement failsalong some shape measurement lines. Therefore, among the measuredflatness values of the latest ten times, the case occurs in which thenumber of times when the measurement is successful is smaller than five,so that the measured value cannot be believed, and cannot be outputtedto the control unit. In particular, the front end of the hot-rolledsteel sheet S, at which the flatness must inherently controlled, cannotbe measured because of the non-tension state. On the other hand, asshown in FIG. 16, in the case where the staggered pattern is used as alight and dark pattern, over almost the overall length of one coil ofthe hot-rolled steel sheet S, not only the edge detection but also themeasurement of surface shape can be made properly, which reveals thatimprovement has been accomplished as compared with the conventionalexample.

<Effect of Determination of Effectiveness>

FIG. 17 shows measurement examples of degree of steepness and the likeover the overall length of one coil of steel sheet in the case where thestaggered pattern of this embodiment is used as a light and dark patternprojected onto the surface of the hot-rolled steel sheet S for thehot-rolled steel sheet S having material quality of low surfacereflectance. FIG. 17A shows the measured values of degree of steepnessmeasured along the shape measurement lines L11 and L15 in the vicinityof both the edges, FIG. 17B shows the number of times when themeasurement was successful among the measured flatness values of thelatest ten times, FIG. 17C shows whether or not the edge of thehot-rolled steel sheet S could be detected, and FIG. 17D shows thenumber of shape measurement lines along which the surface shape could bemeasured properly.

As shown in FIG. 17, in this example, the case somewhat occurs in whichthe edge detection cannot be made (FIG. 17C); however, if among themeasured flatness values of the latest ten times, the number of timeswhen the measurement is successful is not smaller than five, among themeasured flatness values of the latest ten times, the average value ofthe measured flatness values succeeded in measurement is outputted tothe control unit as the effective flatness measurement result, so thatthe flatness measurement result is outputted continuously over theoverall length of one coil of the hot-rolled steel sheet S (FIG. 17B).In some cases, the surface shape could be measured properly for all ofthe five shape measurement lines regardless of the fact that the edgedetection could not be made (locations surrounded by a broken line inFIGS. 17C and 17D). The reason for this is that in the case where it ismistakenly detected that a point on the inside of the true edge of thehot-rolled steel sheet S is the edge, the staggered pattern is projectedon the inside thereof, so that the surface shape can be measuredproperly. This result reveals that the determination as to whether ornot the measured flatness value succeeded in measurement must be made byboth of, not either one of, the determinations as to whether the edge ofthe sheet material could be detected properly and as to whether or notthe surface shape of the sheet material could be measured properly.

<Measurement Stability>

Table 1 gives one example of a result of comparison between themeasurement stability in the case where the conventional linear patternis used and the measurement stability in the case where the staggeredpattern of this embodiment is used for the hot-rolled steel sheet S ofthe same steel type. Since the state of the surface of the hot-rolledsteel sheet S differs depending on the steel type, the measurementstability is compared for a steel type that is the same as the steeltype for which the surface shape measurement success percentage in thecase where the conventional linear pattern is used is on the low side.The edge detection success percentage, the surface shape measurementsuccess percentage, and the effectiveness determination percentage inTable 1 are average values of values determined by Formulas (14) to (16)for each coil of the hot-rolled steel sheet S, respectively. The methodfor detecting the edge and the method for measuring the surface shapeare as described above.Edge detection success percentage=(number of times when edge detectionis successful/number of processed images over overall length of onecoil)×100  (14)Surface shape measurement success percentage=(number of times whensurface shape measurement is successful/number of processed images overoverall length of one coil)×100  (15)Effectiveness determination percentage=(number of times when both ofsurface shape measurement and edge detection are successful/number ofprocessed images over overall length of one coil)×100  (16)

TABLE 1 Surface shape Edge detection measurement EffectivenessProjection Number of success success determination pattern coilspercentage percentage percentage Linear 163 99.9% 83.8% 94.2% patternStaggered 258 99.2% 97.9% 98.6% pattern

Concerning the edge detection, it can be seen that in either case, thesuccess percentage was 99% or more, and there is scarcely a differencebetween the case where the linear pattern is used and the case where thestaggered pattern is used. In other words, it can be said that even ifthe staggered pattern is used as the projection pattern, the edgedetectability does not decrease. Concerning the surface shapemeasurement, the success percentage was 83.8% in the case where theconventional linear pattern was used, whereas the success percentage wassignificantly increased to 97.9% by using the staggered pattern. As theresult, the effectiveness determination percentage was also increasedfrom 94.2% to 98.6%.

As described above, considering that faulty measurement in the casewhere the conventional linear pattern is used occurs frequently in poorflatness portions that should be controlled inherently, it can beexpected to have a great advantage when measured flatness valuesachieved by the use of the staggered pattern as in this embodiment areapplied to the control. Moreover, by turning the control on and offbased on the effectiveness determination of measurement result, mistakencontrol caused by an abnormal measured value can be prevented, so thatsteady control can be realized.

The invention claimed is:
 1. A method for measuring flatness of a sheetmaterial in which a light and dark pattern composed by a light portionand a dark portion is projected onto a surface of the sheet materialrunning in a longitudinal direction, a pattern image is acquired byphotographing the light and dark pattern by an image pickup devicehaving an image pickup visual field larger than a width of the sheetmaterial, and the flatness of the sheet material is measured byanalyzing the acquired pattern image, comprising: a first step of usinga staggered pattern as the light and dark pattern projected onto thesurface of the sheet material, the staggered pattern being composed oflight portions arranged in a staggered form at a predetermined presetpitch in a longitudinal direction and a transverse direction, andprojecting the staggered pattern onto the surface of the sheet materialso that the longitudinal direction of the staggered pattern correspondsto the longitudinal direction of the sheet material and the transversedirection of the staggered pattern corresponds to the width direction ofthe sheet material; a second step of arranging the image pickup deviceat a position at which the specularly reflected light on the surface ofthe sheet material of the staggered pattern can be received, andacquiring the pattern image by photographing the staggered pattern bythe image pickup device; a third step of setting a shape measurementline extending along the longitudinal direction of the staggered patternat a predetermined position in the acquired pattern image; a fourth stepof averaging picture element concentrations on a straight line whichpasses through a picture element on the shape measurement line andextends in the transverse direction of the staggered pattern, and has alength two times or more the transverse preset pitch between the lightportions to calculate an average picture element concentration; a fifthstep of calculating distribution of the average picture elementconcentrations along the shape measurement line; and a sixth step ofcalculating a surface shape of the sheet material along the shapemeasurement line based on the calculated average picture elementconcentration distribution, and calculating the flatness of the sheetmaterial based on the calculated surface shape; wherein the methodfurther comprises a step of, for a reference material which is placed inparallel with the running direction of the sheet material, on whichflatness is measured, and has a flat surface shape, executing the firstto fifth steps to calculate the average picture element concentrationdistribution along the shape measurement line in the pattern imageacquired for the reference material, and based on the average pictureelement concentration distribution, calculating in advance thedistribution p_(s)(x) of longitudinal pitches between light portions ofthe staggered pattern along the shape measurement line in the patternimage acquired for the reference material; and the sixth step comprises:a step of, based on the average picture element concentrationdistribution calculated for the sheet material, calculating thedistribution p_(m)(x) of longitudinal pitches between light portions ofthe staggered pattern along the shape measurement line in the patternimage acquired for the sheet material, and a step of calculating thedistribution θ(x) of the inclination angles of the surface of the sheetmaterial along the shape measurement line based on Formula (1), andcalculating the surface shape of the sheet material based on thedistribution θ(x) of inclination angles of the surface of the sheetmaterial: $\begin{matrix}{{\theta(x)} = {\tan^{- 1}\left\{ \frac{\left( {{p_{m}(x)}/{p_{S}(x)}} \right) - 1}{{\tan\;\alpha} + {\left( {{p_{m}(x)}/{p_{S}(x)}} \right)\tan\;\beta}} \right\}}} & (1)\end{matrix}$ in Formula (1), x denotes a position along thelongitudinal direction of the staggered pattern in the pattern image (aposition along the longitudinal direction of the sheet material), θ(x)denotes the distribution of inclination angles between the sheetmaterial running direction and the surface of sheet material, α denotesthe angle between the direction perpendicular to the sheet materialrunning direction and the image pickup direction of image pickup device,and β denotes the angle between the direction perpendicular to the sheetmaterial running direction and the projection direction of the staggeredpattern.
 2. The method for measuring flatness of a sheet materialaccording to claim 1, wherein the third step comprises: a step ofsetting an edge detection line extending in the transverse direction ofthe staggered pattern at a predetermined position in the acquiredpattern image; a step of calculating, successively along the edgedetection line, standard deviations of picture element concentrations ona straight line which passes through the picture element on the edgedetection line and extends along the longitudinal direction of thestaggered pattern, and has a length two times or more the longitudinalpreset pitch between light portions; a step of, based on the magnitudeof the calculated picture element concentration standard deviation,detecting the picture element corresponding to the edge of the sheetmaterial on the edge detection line; and a step of setting the shapemeasurement line with the detected picture element corresponding to theedge being a reference.
 3. The method for measuring flatness of a sheetmaterial according to claim 1, wherein as the image pickup device, ahigh-sensitivity image pickup device and a low-sensitivity image pickupdevice having a sensitivity lower than that of the high-sensitivityimage pickup device are used; in the second step, the high-sensitivityimage pickup device and the low-sensitivity image pickup device arearranged in parallel so that the image pickup visual fields thereof haveportions overlapping with each other; in the third step, the shapemeasurement line is set at the corresponding position in the patternimages acquired by the high-sensitivity image pickup device and thelow-sensitivity image pickup device respectively; and the sixth stepcomprises: a step of counting the number of picture elements in whichthe concentration saturates, in the average picture elementconcentration distribution along the shape measurement line set in thepattern image acquired by the high-sensitivity image pickup device, anda step of, if the number of concentration saturated picture elements isnot smaller than a preset predefined threshold value, calculating thesurface shape of the sheet material along the shape measurement linebased on the average picture element concentration distribution alongthe shape measurement line set in the pattern image acquired by thelow-sensitivity image pickup device, and if the number of concentrationsaturated picture elements is smaller than the preset threshold value,calculating the surface shape of the sheet material along the shapemeasurement line based on the average picture element concentrationdistribution along the shape measurement line set in the pattern imageacquired by the high-sensitivity image pickup device.
 4. The method formeasuring flatness of a sheet material according to claim 1, wherein themethod further comprises: a seventh step of measuring the flatnesssuccessively in a plurality of different portions in the longitudinaldirection of the sheet material by repeatedly executing the first tosixth steps for the sheet material running in the longitudinaldirection; an eighth step of determining whether or not the measuredflatness values of preset latest N times (N: integer of 2 or more)succeeded in measurement; and a ninth step of, if among the measuredflatness values of the latest N times, the number of times when it isdetermined that the measurement is successful is not smaller than apreset threshold value, generating a signal indicative of success inmeasurement, and outputting the average value of the measured flatnessvalue succeeded in measurement among the measured flatness values of thelatest N times as a flatness measurement result, and if among themeasured flatness values of the latest N times, the number of times whenit is determined that the measurement is successful is smaller than thethreshold value, generating a signal indicative of failure inmeasurement.
 5. The method for measuring flatness of a sheet materialaccording to claim 4, wherein the eighth step comprises: a step ofsetting two edge detection lines extending in the transverse directionof the staggered pattern at different positions in the longitudinaldirection of the staggered pattern in each of the pattern images used toobtain the measured flatness values of the latest N times; a step ofdetecting the picture element corresponding to the edge of the sheetmaterial on each of the edge detection lines; and a step of, based on acoordinate of the detected picture element corresponding to the edge ofthe sheet material and the amplitude of the average picture elementconcentration distribution along the shape measurement line calculatedin the fifth step, determining whether or not the measured flatnessvalues of the latest N times succeeded in measurement.
 6. A method forproducing a steel sheet in which method a steel sheet is produced byrolling a slab, which is roughly rolled by a roughing mill, by using afinishing mill train, and thereafter by being cooled by a cooling zone,wherein the flatness of the steel sheet is measured by the flatnessmeasuring method described in claim 1, and based on the measurementresult, rolling conditions of the finishing mill or cooling conditionsof the cooling zone are controlled.